High-density pressurelessly sintered zirconium diboride/silicon carbide composite bodies and a method for producing the same

ABSTRACT

A method of sintering a ZrB 2 —SiC composite body at ambient pressures, including blending a first predetermined amount of ZrB 2  powder with a second predetermined amount of SiC powder, wherein both powders are characterized by the presence of surface oxide impurities. Next the blended powders are mixed to yield a substantially homogeneous powder mixture and a portion of the substantially homogeneous powder mixture is formed into a green body. The body is fired to a first temperature, wherein substantially all surface oxide impurities are reduced and/or volatilized to substantially eliminate oxides from the green body, and the body is heated to a second temperature and sintered to yield a composite body of at least about 99 percent theoretical density (more typically at least about 99.5 percent theoretical density) and characterized by SiC whisker-like inclusions distributed substantially evenly in a ZrB 2  matrix.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation-in-part of and claims priorityto U.S. patent application Ser. No. 11/419,622 filed on May 22, 2006,now abandoned.

TECHNICAL FIELD

The novel technology herein relates generally to the field of zirconiumdiboride ceramics, and, more particularly, to a pressurelessly sinteredprimarily zirconium diboride ceramic body and a method for making thesame.

BACKGROUND

Traditionally, zirconium boride and composites thereof, such aszirconium boride/silicon carbide composites, have been fabricated by ahot pressing process. Mixtures of zirconium boride and silicon carbidepowders are placed in a pressure vessel and are subjected to elevatedpressures while heated to high temperatures, typically in an inertatmosphere or under vacuum. Alternately, ZrB₂/SiC composites may beformed by reaction hot pressing precursors such as metallic Zr, Sipowders and boron carbide (B₄C) powder (instead of SiC and ZrB₂ powderprecursors). In either technique, the lack of self-diffusion and lowdriving forces for sintering and densification inherent in the materialsis compensated for through the application of high pressures during thesintering step. The high pressures applied to the sintering bodycontribute sufficient forces such that substantially completedensification of the sintering body may be achieved.

Typically, substantially dense composite bodies are formed as follows.First, the raw material powders are blended and then loaded into asimple geometrical model, such as a graphite die, where the blended rawmaterials then undergo heating and pressing simultaneously. Although hotpressing is not required per se for the sintering of ZrB₂/SiCcomposites, sintering without the application of elevated pressuresresults in weak bodies characterized by densities only about 90 percentof theoretical and having poor thermal and mechanical properties.Therefore, the densified bodies so produced are limited by theconstraints of the hot pressing die to simple shapes and moderate sizes.Further, hot pressing techniques require expensive hot pressingfacilities and provide a slow rate of production. Moreover, the bodiesproduced by hot pressing techniques are simple and unfinished, thustypically requiring further diamond machining in order to produce afinished end product. Such machining adds considerable time andfinancial cost.

In the hot pressing processes, the attendant high pressures arenecessary to provide sufficient driving force for substantialdensification to occur, since the mixed ZrB₂ and SiC powders alone lacksufficient self-diffusion characteristics when heated to sinteringtemperatures. The use of high sintering pressures addresses this problemby providing an externally generated driving force to the system, butalso adds complexity and cost to the fabrication of ZrB₂/SiC bodies.Further, the application of high pressure adds inherent geometricalconstraints that limit the bodies so formed to simple geometric shapes.Thus, there remains a need for a means of fabricating and sinteringZrB₂/SiC bodies having complex shapes at ambient pressures. The presentnovel technology addresses this need.

SUMMARY

The present novel technology relates to a method of sintering ZrB₂/SiCbodies at ambient pressures, as well as to control of the microstructureof the so-produced ZrB₂/SiC bodies.

One object of the present novel technology is to provide an improvedmethod for producing ZrB₂/SiC bodies. Related objects and advantages ofthe present novel technology will be apparent from the followingdescription.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a method for sintering aZrB₂—SiC composite body to substantially full density without theapplication of high pressures according to a first embodiment of thepresent novel technology.

FIG. 2 is a photomicrograph of one embodiment of the present noveltechnology, a ZrB₂—SiC composite body characterized by SiC whisker-likeinclusions substantially evenly distributed in a ZrB₂ matrix.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the purposes of promoting an understanding of the principles of thenovel technology and presenting its currently understood best mode ofoperation, reference will now be made to the embodiments illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of thenovel technology is thereby intended, with such alterations and furthermodifications in the illustrated device and such further applications ofthe principles of the novel technology as illustrated therein beingcontemplated as would normally occur to one skilled in the art to whichthe novel technology relates.

Densified ZrB₂/SiC composites are attractive as ultra-high melting pointmaterials that are also characterized as having high strength andhardness, as well as being chemically stable and having relatively highthermal and electrical conductivities. This combination of properties israre, and thus ZrB₂/SiC composites are desired for applications in theaerospace field, as well as in electrode, cutting tool, machining tool,and molten metal containing crucible applications and the like.

Although ZrB₂/SiC composite materials may be readily formed and sinteredfrom a combination of the appropriate amounts of blended ZrB₂ and SiCpowders (or reacted from precursors such as Zr, Si and boron carbide), asignificant amount of oxide impurities, especially SiO₂ and ZrO₂impurities, are always present as surface impurities in the powdersand/or precursor materials. The presence of these oxides promotesnon-densifying coarsening mechanisms among the ZrB₂ and SiC particles,resulting in a practical upper limit of densification of sinteredZrB₂/SiC composites of about 93 percent of theoretical density, thusyielding porous ZrB₂/SiC composite bodies with relatively large grainsizes that have only a fraction of the strength, hardness, and likephysical and chemical properties that make the composite materialsattractive and useful.

Instead of overcoming the low self-diffusion and low driving force fordensification inherent in the ZrB₂/SiC composites materials through theapplication of high sintering pressures, the present technique involvesreduction and removal of the oxide impurities themselves early in thesintering process. This may be accomplished, for example, through theaddition of one or more reducing agents to the Zr- and Si-containing rawmaterials, such as boron carbide (B₄C), loosely bound or free carbon insufficient quantities to convert SiO₂ plus C into SiC plus CO₂ and toconvert ZrO₂ plus B₄C into ZrB₂ plus CO₂. Other gaseous reactionproducts, such as CO, SiO and B₂O₃ vapor, may also be produced andevolved. These reactions occur at moderately elevated temperatures,wherein the CO₂ and other gases so evolved escape the bodies and thusallows for the remaining ZrB₂ and SiC grains to sinter to substantiallyfull theoretical density, typically at least about 99.5 percenttheoretical. Typically, at least one of the reducing agents is typicallyfree carbon, an organic compound from which carbon is easily releasedupon heating (such as phenolic resin), or graphite, or may be a carbidematerial such as B₄C, WC, ZrC, HfC, Mo₂C, NbC or the like.

FIG. 1 illustrates the process for sintering ZrB₂—SiC composite bodiesat ambient pressures. In operation, a pressurelessly sintered ZrB₂composite body 10 substantially free of oxide impurities and typicallyhaving a composition of between about 1 weight percent to about 40weight percent SiC with the rest being a substantially ZrB₂ matrix maybe produced as follows. First, the Zr-, Si- and B-source powders 20, 22,24 (typically ZrB₂, and SiC, but alternately, Zr and Si metal and B₄C,or the like), along with smaller amounts of reducing agents 23(typically C-containing additives such as organic resins and compounds,elemental C, B₄C, and other like refractory carbides) and binders 25,are measured in predetermined amounts and then mixed. Typically, betweenabout 2 weight percent and about 25 weight percent SiC, between about0.1 and about 4 weight percent reducing agent, and, if necessary,between about 1 weight percent and about 5 weight percent bindermaterial are mixed with ZrB₂ to produce a substantially homogeneouslyblended powder precursor mixture 26. In some instances, the reducingagent 23 may also act as a binder 25, such as when the reducing agent 23is a phenolic resin material. Typically, an organic solvent 28 is addedto the mixed powders to form a suspension 30, which may then be furthermixed, such as by ball milling 32, to form a substantially homogeneousslurry 34. The slurry 34 may then be dried and a substantiallyhomogenous mixed powder precursor 40 may be recovered.

A portion of the substantially homogeneously blended powder mixture isthen separated and formed into a green body 42. If binders and/or resinsare present in the green body, the green body is first heated to atemperature sufficient for the binder to volatilize 44, such as about400 to about 600 degrees Celsius. Binder burnout and/or resincarbonization are typically accomplished in an inert atmosphere.

Next, the temperature is elevated and the green body is “soaked” orallowed to remain at one or more elevated temperatures 46 (such as about1650 degrees Celsius) for sufficient time for any B₂O₃ to volatilize andfor the other oxide impurities to react with the present reducing agents(typically carbon or carbon compounds) to produce reaction products(typically carbon dioxide gas and other reaction products); this istypically done in a very low oxygen partial pressure atmosphere such asa flowing, non-oxide gas (such as H₂, He, argon, or similar gasmixtures), and more typically in a vacuum or partial vacuum (toencourage evolution and removal of carbon dioxide gas) to produce anoxide-reduced or partially-sintered body. The temperature of the reducedor partially-sintered body is then raised to a temperature sufficientfor substantially complete densification to occur in a matter of hours(such as to about 2100 degrees Celsius) 48. The body is then soaked atthe elevated temperature for a time sufficient for substantially fulldensification to occur (such as a temperature of about 2100 degreesCelsius for 2 to 4 hours) to yield a substantially theoretically densesintered body 10. This final soak is usually done in an inert gasatmosphere.

In one embodiment, a powder system is defined as having a compositionalrange of between about 2 and about 25 wt. percent SiC with the remainderbeing ZrB₂. Free carbon (typically about 2 wt. percent) is added to thesystem, typically via dissolved phenolic resin as a carbon precursor, toeffectively remove any SiO₂, ZrO₂ and/or other oxide impurities that maybe present. Typically, a small amount of B₄C is also added to thesystem, such as between about 2 and about 4 wt. percent. More typically,a small amount (typically about 0 to 4 wt. percent) of binder (such aspolypropylene carbonate) is likewise added to enhance the pressabilityof the material.

Typically, fine α-SiC, B₄C and as-received ZrB₂ powders in designedvolume or mass fraction are dispersed in a non-aqueous solvent 28, suchas Methyl Ethyl Ketone (MEK). The suspension 30 is typically mixed 32,such as by ball milling, planetary mixing, or attrition milling for apredetermined amount of time (typically about 24 hours for ball millingwith WC milling media). A free carbon source 23, such as 2 wt. percentphenolic resin, based on the total weight of ZrB₂ and SiC, is added tothe mixture followed by further mixing 32 (such as ball milling for anadditional 24 hours). If mixing was done in slurry form, the slurry isthen dried 36 to yield a powder mixture 40. The powder mixture 40 istypically ground and sieved to yield agglomerates of the powder mixture.This could also be accomplished by a spray drying technique. Theagglomerates are then formed into green bodies 44, such as by uni-axialpressing and/or cold isostatic pressing (CIP) in molds of a desiredshape. Pressing 38 is typically done at 40-50 Kpsi. The green bodies 44may alternately be formed through other known techniques, such as viainjection molding, extrusion, slip casting or the like to produce morecomplex shapes by those skilled in the art.

The green bodies 44 typically undergo binder burnout/resin carbonization42 through exposure to sufficiently elevated temperatures in a lowoxygen or inert gas atmosphere for sufficient time to substantiallycompletely volatilize the present binder material (such as in flowing Arat 400 degrees Celsius to about 600 degrees Celsius for 2-4 hours).Binder burnout/resin carbonization 42 is typically followed bypressureless sintering 45 (more typically in a graphite furnace) at asufficiently elevated temperature (typically at least about 2050 degreesCelsius) for a time sufficient to achieve theoretical ornear-theoretical density (such as about 4 hours at 2050 degreesCelsius).

The sintering process 45 is more typically divided into two stages 46,48. The first stage 46 is a reaction period that may be defined as thetemperature range from room temperature to about 1650 degrees Celsiusunder vacuum. In this stage 46, oxide impurities are removed from thesystem. Once the oxide impurities are substantially removed from thesystem, the second stage may be initiated. The second stage 48 is asintering period that may be typically defined by the temperature rangefrom about 1650 degrees Celsius to the final sintering temperature(typically about 2050° C. or higher). The second stage 48 typicallyoccurs in the presence of an inert gas atmosphere at ambient pressures,such as one provided by flowing Ar.

After the second stage 48 is complete, the sintered bodies 10 havesubstantially achieved near theoretical density. Further, themicrostructure of the sintered bodies 10 can be varied such that themorphology of the SiC particles can be more or less “whisker like” α-SiCinclusions 50 (i.e., the α-SiC inclusions 50 may have shapes rangingfrom acicular to equiaxial) that are uniformly distributed in a ZrB₂matrix 52. FIG. 2 shows a ZrB2 matrix 52 having substantially uniformlydispersed α-SiC whisker-like inclusions 50 therein. In this example, theSiC inclusions 50 are typically between 15 and 20 μm in length, and aremore generally about 20 μm in length.

According to one aspect of the present novel technology, a method ofproducing substantially dense ZrB₂—SiC composite materials 10 withoutthe use of applied pressures during sintering, or otherwise hotpressing, generally includes the steps of:

(a) mixing between about 2 weight percent and about 25 weight percentSiC powder 22, between about 0.1 and about 4 weight percent reducingagent 23, and about 1 weight percent to about 5 weight percent bindermaterial 25 with ZrB₂ powder 20 to produce a substantially homogeneouslyblended powder precursor mixture 40, wherein oxide impurities arepresent in the SiC and ZrB₂ powders 22, 20;

(b) forming 38 a portion of the substantially homogeneously blendedpowder precursor mixture into a green body 44;

(c) heating 42 the green body 44 to a temperature in the range of about400 degrees Celsius to about 600 degrees Celsius in an inert atmospherefor a sufficient time to substantially remove the binder/resin 25;

(d) substantially reducing 44 oxide impurities present in the green bodyby heating under vacuum;

(e) placing the green body 44 in an inert gas atmosphere and elevatingthe temperature of the green body to a temperature sufficient forsintering to progress; and

(f) soaking 46 the green body 44 in an inert gas atmosphere at atemperature sufficient for sintering to progress for sufficient time toyield a substantially theoretically dense sintered body 10 (i.e., a body10 having very low total or absolute porosity, typically less than 2percent, more typically less than 1 percent, and still more typicallyless than 0.5 percent; in other words, the body 10 has a very highdensity, typically in excess of 98 percent theoretical, more typicallyin excess of 99 percent theoretical, and still more typically in excessof 99.5 percent theoretical).

As detailed above, the reducing agent 23 is typically B₄C and/or a freecarbon additive, such as carbon black or phenolic resin, added duringthe powder precursor blending/mixing step 32. SiC 22 is typicallypresent in an amount from about 2 weight percent to about 25 weightpercent, and is more typically present in an amount from about 5 weightpercent to about 20 weight percent.

Further, while step (f) above could be performed under elevatedpressures, such as in a hot isostatic press, such pressures areunnecessary if the level of oxide impurities present in the green bodyis sufficiently reduced.

EXAMPLE 1

A ZrB₂—SiC composite composition may be formed as having 20 weightpercent SiC, 3 weight percent carbon derived from phenolic resin (whichalso acts as a binder), and the remainder ZrB₂. The composition may bedispersed in a MEK liquid medium and ball milled for 24 hours with WCmedia so as to be thoroughly mixed. The mixed slurry may be dried toyield a mixed powder, and the recovered powder may be ground and sievedto a predetermined desired particle size distribution. A portion of thesieved powder may then be formed into a green body via uniaxial pressingfollowed by cold isostatic pressing. The green body may then be heatedto about 600 degrees Celsius in flowing argon and held at thattemperature for 4 hours to volatilize and evolve gasses produced throughresin decomposition. The green body may then be heated to 1650 degreesCelsius in a partial vacuum and held there for up to 6 hours tovolatilize boron oxides and react any other oxide impurities with thereducing agent; such impurities are reduced by the carbon and formrefractory compounds such as ZrC or leave the green body as evolvedgases such as SiO, CO₂ and CO gas. The green body (now the reduced bodyor partially sintered body) is then heated to 2100 degrees Celsius inflowing Argon and held there for 4 hours to yield a substantiallytheoretically dense sintered ZrB₂—SiC composite body with whisker-likeSiC inclusions in a ZrB₂ matrix.

EXAMPLE 2

A ZrB₂—SiC composite precursor composition may be formed as having 15weight percent SiC, 2 weight percent carbon black, 2 weight percentorganic binder, 3 weight percent B₄C, and the remainder ZrB₂. Thestarting composition may be dispersed in a MEK liquid medium and ballmilled for 24 hours with WC media so as to be thoroughly mixed. Theslurry of the mixed powders may be dried to yield a mixed powder withbinder, and the recovered powder may be ground and sieved to apredetermined desired granule size distribution. A portion of the sievedgranules may then be formed into a green body via cold isostaticpressing. The green body may then be heated to about 400 degrees Celsiusin flowing argon and held at that temperature for 4 hours to decomposeand volatilize the binder. The green body may then be heated to 1650degrees Celsius in a partial vacuum and held there for 4 hours to removevolatile boron oxides and to react the remaining oxide impurities withthe carbon and B₄C; such impurities are reduced by the carbon and/or B₄Cto form refractory compounds such as ZrB₂ and ZrC or leave the greenbody as evolved gases such as SiO, CO₂ and CO gas. The green body (nowthe reduced body or partially sintered body) is then heated to 2050degrees Celsius in flowing Argon and held there for 4 hours to yield asintered ZrB₂—SiC composite body having a porosity of less than 0.5percent/density of greater than 99.5 percent of theoretical andcontaining substantially evenly dispersed SiC particles in a ZrB₂matrix.

EXAMPLE 3

A ZrB₂—SiC composite composition may be formed as having 10 weightpercent SiC, 3 weight percent carbon, 2 weight percent organic binder,and the remainder ZrB₂. The initial composition may be dispersed in aMEK liquid medium and ball milled for 24 hours with WC media so as to bethoroughly mixed. The slurry of the mixed powders may be dried to yielda mixed powder with binder, and the recovered powder may be ground andsieved to a predetermined desired granule size distribution. A portionof the sieved granules may then be formed into a green body via coldisostatic pressing. The green body may then be heated to about 350degrees Celsius in flowing argon and held at that temperature for 4hours to decompose and volatilize the binder. The green body may then beheated to 1650 degrees Celsius in a partial vacuum and held there for 6hours to volatilize any boron oxides and to react any other oxideimpurities with the reducing agent additive; such impurities are reducedby the additive to form refractory compounds such as ZrB₂ or ZrC orleave the green body as evolved SiO, CO₂ and CO gas. The green body (nowthe reduced body or partially sintered body) is then heated to 2050degrees Celsius in flowing Argon and held there for 4 hours to yield asintered ZrB₂—SiC composite body with a total porosity of less thanabout 1 percent.

EXAMPLE 4

A ZrB₂—SiC composite composition may be formed from the power mixture of19.1 weight percent Si, 18.8 weight percent B₄C, and 62.1 weight percentZr. In addition to the above starting powders, 4 weight percent carbonblack and 2 weight percent organic binder may be added, based on thetotal weight of the combined starting powders. The composition may bedispersed in hexane liquid medium and ball milled for 24 hours with WCmedia so as to be thoroughly mixed. The slurry of the mixed powders maybe dried to yield a mixed powder with organic binders, and the recoveredpowder may be ground and sieved to a predetermined desired granule sizedistribution. A portion of the sieved granules may then be formed into agreen body via uniaxial pressing followed by cold isostatic pressing.The green body may then be heated to about 350 degrees Celsius inflowing argon and held at that temperature for 4 hours to decompose andvolatilize the binder. The green body may then be heated to 1650 degreesCelsius in a partial vacuum and held there for 6 hours to react the Si,Zr, and B₄C. Any present B₂O₃ should volatilize under these conditions.Likewise, other oxide impurities, such as ZrO₂ and/or SiO, should reactwith the present carbon reducing agent additive to form refractorycompounds such as ZrC and/or SiC or leave the green body as evolved SiO,CO₂ and CO gas. The green body (now the reduced body or partiallysintered body) is then heated to 2100 degrees Celsius in flowing Argonand held there for 4 hours to yield a substantially theoretically densesintered ZrB₂—SiC composite body.

EXAMPLE 5

A ZrB₂—SiC composite composition may be formed as having 20 weightpercent SiC, 2 weight percent phenolic resin, 15 weight percent WC (suchan addition may be made intentionally as a powder or as a calculatedlevel of WC imparted during particle size reduction and mixing steps),and the remainder ZrB₂. The ZrB₂ powder may be reduced in size byattrition milling using WC media and a WC spindle. The milled ZrB₂powder would then be mixed with the SiC and phenolic resin by dispersingin a hexane liquid medium and ball milled for 24 hours with WC media soas to be thoroughly mixed. The slurry of the mixed powders may be driedto yield a mixed powder with resin, and the recovered powder may beground and sieved to a predetermined desired granule size distribution.A portion of the sieved granules may then be formed into a green bodyvia uniaxial pressing followed by cold isostatic pressing. The greenbody may then be heated to about 500 degrees Celsius in flowing argonand held at that temperature for 4 hours to carbonize the resin. Thegreen body may then be heated to 1450 degrees Celsius in a partialvacuum and held there for 6 hours to volatilize any boron oxides andthen heated to 1850 degrees Celsius and held there for 6 hours to reactany other oxide impurities with the reducing agent additives; suchimpurities are reduced by the additive to form refractory compounds suchas ZrC and/or W-containing solid solutions of those compounds or leavethe green body as evolved W-oxides, SiO, CO₂ and CO gas. The green body(now the reduced body or partially sintered body) is then heated to 2050degrees Celsius in flowing Argon and held there for 4 hours to yield asubstantially theoretically dense sintered ZrB₂—SiC composite body.

While the novel technology has been illustrated and described in detailin the drawings and foregoing description, the same is to be consideredas illustrative and not restrictive in character. It is understood thatthe embodiments have been shown and described in the foregoingspecification in satisfaction of the best mode and enablementrequirements. It is understood that one of ordinary skill in the artcould readily make a nigh-infinite number of insubstantial changes andmodifications to the above-described embodiments and that it would beimpractical to attempt to describe all such embodiment variations in thepresent specification. Accordingly, it is understood that all changesand modifications that come within the spirit of the novel technologyare desired to be protected.

1. An oxide-free zirconium diboride composite body system, comprising acomposite body including from about 1 weight percent to about 25 weightpercent SiC whiskers substantially homogeneously distributed in a ZrB₂matrix, an ambient temperature of about 2050 degrees Celsius, and aflowing inert gas environment enveloping the composite body, wherein thecomposite body is free of oxygen.
 2. A ZrB₂ composite system,comprising: a ZrB₂ matrix; between about 1 weight percent and about 25weight percent SiC particles substantially homogeneously distributed inthe ZrB₂ matrix and defining a composite body; an ambient temperature ofabout 2050 degrees Celsius; an ambient pressure of about 1 atmosphere;wherein the composite body is in an oxygen free environment; wherein thecomposite body is free of oxygen; and wherein the composite body is morethan 99.5 theoretically dense.
 3. The ZrB₂ composite system of claim 2wherein the oxygen free environment is an inert atmosphere.
 4. The ZrB₂composite system of claim 2 wherein the oxygen free environment is areducing atmosphere.
 5. The ZrB₂ composite system of claim 2 wherein theSiC particles are whiskers.